A plurality of fins comprising thin sheets are provided on outer surfaces of tubes which are members constituting a heat exchanger such as a radiator for a motercycle or an automobile, or a condenser for a cooler, in order to increase the radiating surface area of the tube. When a tube is made of copper, aluminum or alloys thereof, it is the usual practice to use, as the fin material, sheets of 1050 aluminum (aluminum with a purity of at least 99.5%) of the AA Standard (the Standards of the Aluminum Association), 3003 alloy (Al-1.2% Mn alloy) of said Standard, or 7072 alloy (Al-1.2% Zn alloy) of said Standard, or clad sheets manufactured by applying a brazing metal sheet to one or both sides of these sheets.
Various methods are available for installing fins of any of the aforementioned materials on the outer surface of a tube of any of the aforementioned materials. Among such various methods, the vacuum brazing method for brazing many joints at a time is widely adopted in the industry, which comprises: tacking a plurality of fins substantially vertically at prescribed intervals onto the outer surface of a tube by the use of an appropriate jig to form an assembly, at least one of the outer surface of said tube, and the both surfaces or one surface of said fin being previously applied with a brazing metal sheet; heating said assembly to a temperature of from about 580 to about 620.degree. C. in a vacuum furnace to melt said brazing metal, thereby brazing said plurality of fins to the outer surface of said tube with said melted brazing metal at a time by a single heating.
In general, when a heat exchanger comprising tubes made of aluminum or an aluminum alloy is used in a corrding environment such as a wet atmosphere, the tubes are rapidly corroded, thus reducing the service life of the heat exchanger. In such a case, therefore, the 7072 alloy (Al-1.2% Zn alloy) of AA Standard mentioned above having a relatively high sacrificial anode property is most commonly employed as the fin material to provide the tube with a cathodic protection with fins as sacrificial anodes.
However, when fins of the 7072 alloy of AA Standard are brazed to the outer surface of the tube by the aforementioned vacuum brazing method, the fins sag down and deform under the effect of external forces and their own weight, because of the low high-temperature sagging resistance of the 7072 alloy of AA Standard, and as a result, the fins cannot hold their original shape upon forming the assembly. In addition, the vacuum atmosphere and the high temperature involved during the vacuum brazing cause a considerable portion of the zinc content of the 7072 alloy of AA Standard to evaporate. When manufacturing a large-capacity heat exchanger, in particular, the necessity of holding a high temperature for a long period at time leads to a very large amount of evaporation of the zinc content. Accordingly, the sacrificial anode property of the fin is seriously impaired or even lost, resulting in corroded tubes in service, this often reducing the service life of the heat exchanger.